Now when they heard these things they were enraged, and they ground their teeth at him. But he, full of the Holy Spirit, gazed into heaven and saw the glory of God, and Jesus standing at the right hand of God. And he said, “Behold, I see the heavens opened, and the Son of Man standing at the right hand of God.” But they cried out with a loud voice and stopped their ears and rushed together at him. Then they cast him out of the city and stoned him. And the witnesses laid down their garments at the feet of a young man named Saul. And as they were stoning Stephen, he called out, “Lord Jesus, receive my spirit.” And falling to his knees he cried out with a loud voice, “Lord, do not hold this sin against them.” And when he had said this, he fell asleep. (Acts 7:54-60, ESV)
Stephen sees a vision of the glory of God with Jesus standing at the right hand of God. Jesus normally sits at the right hand of God. His work is completed and he sits down at the throne, the sitting is significant. So it is also significant that he stands up as Stephen is about to die. Jesus was not absent when Stephen was stoned, this was not outside Jesus power or concern. Stephen was about to go to Jesus, and Jesus stands.
"He stands ready to receive him and crown him, and in the mean time to give him a prospect of the joy set before him" Matthew Henry
"He has stood to receive Stephen’s testimony and to be his advocate. He has stood that he might come forward to be the judge of those who will trample upon God’s prophet. Jesus is rising from his throne to come to Stephen’s defense and to judge his persecutors." Kevin Deyoung